The Prior Art
The use of magnetic flux leakage inspection tools in pipelines is an established technology. Flaws in ferromagnetic pipes have been detected by establishing a magnetic field in the wall of the pipe and detecting flux leakage caused by anomalies in the pipe wall. Distortion of the magnetic field caused by anomalies such as corrosion, pits, or changes in the structure of the pipe wall, some of which can be caused by couplings, welds, collars, or so forth, can be found, measured, and identified in recorded information. Flux leakage pipeline inspection tools therefore provide an established method of determining the characteristics of a pipeline through which an inspection pig passes.
One problem which has existed with reference to flux leakage inspection tools is that of identifying whether a detected flux leakage is occasioned by anomalies, such as corrosion, on the interior or exterior pipe wall surface. When making an inspection of a pipeline it is important to record the location and size of anomalies in the pipe wall as well as of the anomaly identified as to whether it exists on the pipe interior surface or the pipe exterior surface.
A known means of measuring flux leakage is by the use of a Hall-effect device.
Flux leakage inspection instrument pigs typically include the use of a plurality of armatures, each having at one end a positive magnetic pole and at the other end a negative magnetic pole. The magnets are constructed and dimensioned so as to substantially magnetically saturate a short circumferential length of the pipe as the inspection pig moves through the pipe.
For background information relating to instrument pig used for pipeline inspection and particularly pigs that detect anomalies by measuring flux leakage, reference can be made to the following previously issued United States Patents:
PATENTDATE OFNUMBERISSUETITLE 3,949,292Apr. 6, 1976Pipeline Inspection Device with Pivotal Sup-port Structure4,769,598Sep. 6, 1988Apparatus for Electromagnetically Testingthe Walls of Pipelines4,945,306Jul. 31, 1990Coil and Hall Device Circuit for SensingMagnetic Fields4,964,059Oct. 16, 1990Apparatus for Inspecting A Pipeline5,283,520Feb. 1, 1994Method of Determining Thickness ofMagnetic Pipe by Measuring the Time ItTakes the Pipe To Reach MagneticSaturation5,293,117Mar. 8, 1994Magnetic Flaw Detector for Use with Ferro-magnetic Small Diameter Tubular GoodsUsing A Second Magnetic Field To ConfineA First Magnetic Field5,506,505Apr. 9, 1996Apparatus for Remotely Indicating PipelinePig Including A Sensor Housing Having Sur-face Engaging Orthogonally Disposed Para-magnetic Materials A Solid State Sensor andA Flag5,565,633Oct. 15, 1996Spiral Tractor Apparatus and Method5,864,232Jan. 26, 1999Magnetic Flux Pipe Inspection Apparatus forAnalyzing Anomalies In A Pipeline Wall6,023,986Feb. 15, 2000Magnetic Flux Leakage Inspection Tool forPipelines6,640,655Nov. 4, 2003Self Tracking Sensor Suspension Mechanism6,683,452Jan. 27, 2004Magnetic Flux Density Apparatus for, e.g.,Detecting An Internal Crack of A Metal or AShape of the Metal
In addition to the above-listed patents, a relevant reference is an article published in the Society of Petroleum Engineers in May 1991 entitled “Full-Signature Multiple-Channel Vertilog,” G. W. Adams and W. D. Moffat, authors.